H2 Removal via Selective Combustion with O2 During Alkane Dehydrogenation
and Aromatization on Modified ZMS5 Catalysts
Toshio Waku, Joseph A. Biscardi1, Sara Y. Yu, Enrique Iglesia
Department of Chemical Engineering, University of California at Berkeley, Berkeley,
CA, USA. 1ChevronTexaco Research and Technology Co., Richmond, CA, USA.
High H2 concentrations limit unsaturated product yields in alkane reactions by
increasing reverse reaction rates and the hydrogen content of adsorbed reactive
intermediates required for alkene and arene formation. The thermodynamic and
kinetic hurdles imposed by the H2 formed in dehydrogenation and aromatization
reactions can be overcome by selective permeation or chemical reactions of H2. Pdbased membranes [1] and selective H2 combustion catalysts [2] have been used in
order to increase alkene yields in dehydrogenation reactions. Other H2 scavengers,
such as CO, CO2 [3], and thiophene [4] have also been reported. O2 is an attractive
co-reactant because it forms H2O in exothermic reactions that provide the enthalpy
required for autothermal dehydrogenation reactors. The introduction of O2 coreactants at the reactor inlet can lead to explosive mixtures and to undesired
combustion of hydrocarbons. Staging O2 co-reactants in order to match the rate of H2
formation of H2 along tubular reactors ensures the presence of O2 only when H2 is
also available. This approach is difficult to explore with tubular reactors, because it
requires multiple injectors or membranes with axial permeability gradients. Instead,
one can exploit the rigorous equivalence between gradientless semi-batch reactors
with continuous O2 addition and plug-flow reactors with axially distributed O2 feeds.
Here, we use this approach to demonstrate the extent to which unsaturated product
yields can be improved using optimal O2 staging strategies in C3H8 aromatization on
cation-modified H-[Al]ZSM5 and in C3H8 dehydrogenation on Pt/[Fe]ZSM5.
The replacement of Al3+ with Fe3+ in ZSM5 leads to weaker acid sites, which
minimize oligomerization and cracking side reactions [5]. The exchange of 0.1% wt.
Pt onto Na-exchanged [Fe]ZSM5 led to equilibrium C3H6 yields (31 %) at 773 K, 20
kPa C3H8, and 25.8 mol C3H8 / g-atom Pt-s space velocity, with 98% C3H6 selectivity
and no detectable deactivation for >100 h in a tubular flow reactor. Figure 1 shows
propene site yields in a gradientless batch reactor at 723 K. Equilibrium propene
yields are quickly reached as H2 accumulates. The addition of O2 (4 kPa) to C3H8 (20
kPa) leads to the immediate consumption of O2 to form COx and H2O, as a result of
the substantial absence of H2 at low C3H8 conversions. Indeed, COx yields increase
immediately and then remained unchanged, because of the rapid depletion of O2 coreactants in unselective reactions. Higher C3H8 yields are achieved when O2 is
introduced gradually at the rate required in order to consume all of the H2 formed as
the reaction proceeds (Figure 1). In this case, O2 is also immediately consumed upon
introduction, but 89-93% of the O2 added is used to combust H2; in contrast, this H2
combustion selectivity is only 5-8% when O2 is added with the C3H8 reactants. A
systematic study of the effects of O2 introduction rates showed that optimum C3H6
yields and O2 selectivities require stoichiometric feed rates, in which O2 is varied in
order to match the rate of C3H8 dehydrogenation.
2000
Propene
C3H8 / O2 (staged)
1500
C3H8
1000
C3H8 / O2 (co-feed)
Equilibrium C3H6 yield
COX
500
C3H8 / O2 (co-feed)
C3H8 / O2 (staged)
0
0
5
10
15
20
Contact time ks.
Figure 1: The effect of O2 introduction on
C3H6 and COX yields during dehydrogenation
reactions of C3H8 at 723K
References
1.
2.
3.
4.
5.
CC6+
6+ aromatics
Aromatics/ (C1+C2)
/ (C1+C2)selectivity
selectivity ratio
ratio
C 3CO
H8 Xturnovers
mol/Pt-mo
C3H6 and
Yields (mol
C3H8/g-atom Pt)
Similar studies were carried out for C3H8 dehydrocyclodimerization on
H[Al]ZSM5 with and without exchanged cations (Ga, Zn). We discuss here the
specific effects of staged O2 addition on 1.8 wt% Ga/H[Al]ZSM5 at 773K. This
reaction proceeds via a sequence of dehydrogenation steps with intervening
oligomerization, cracking, and cyclization events [3]. In this case, reaction rates
were only weakly influenced by H2 combustion, but the gradual introduction of O2
led to a marked increase in the aromatization to cracking selectivity ratio (Figure 2),
especially as C3H8 conversion increases with increasing contact time. The maximum
attainable C6+ aromatics selectivity increased from 54% with pure C3H8 reactants to
>70% when O2 was introduced as conversion increased. About 95% of the O2
introduced was converted to H2O. The addition of an equivalent O2 amount along
with the C3H8 reactants led to an initial increase in aromatics selectivity (Figure 2)
but to the rapid depletion of O2 in undesired hydrocarbon combustion reactions. As
H2 concentrations then increase with increasing C3H8 conversion, the added O2 is no
longer available for the selective combustion of H2.
This study illustrates the use of a novel protocol for the rigorous simulation and
testing of H2 removal strategies in laboratory reactors. It also shows the feasibility of
selective H2 combustion and the requirement that H2 and O2 co-exist throughout the
reactor in order to ensure complete O2 depletion via H2 combustion reactions, instead
of unselective reactions between O2 and organic reactants and products.
5
C3H8 / O2 (staged)
4
3
C3H8
2
C3H8 / O2 (co-feed)
1
0
0%
0
20%
20
40%
40
60%
60
80%
80
100%
100
to HCtoConversion
PropanePropane
conversion
hydrocarbons (%)
Figure 2: The effect of O2 introduction
on selectivities during dehydrocyclodimerization reactions of C3H8 at 773K
N. Itoh, AIChE J. 33 (1987) 1576.
R.K. Grasselli, D.L. Stern, and J.G. Tsikoyiannis, Appl. Catal. 189 (1999) 9.
E. Iglesia and J.E. Baumgartner, Catal. Lett., 21 (1993) 55.
S.Y. Yu, W. Li, E. Iglesia, J. Catal. 187 (1999) 257.
O. Kresnawahjuesa, G.H. Kuhl, E.J. Gorte, and C.A. Quierini, J. Catal. 210 (2002) 106.